The present invention relates to a method for molding a fused quartz glass block or, more particularly, relates to a method for shaping a fused quartz glass, which may be either obtained from natural quartz or obtained synthetically from a volatile silicon compound such as silicon tetrachloride and silanes, by the plastic deformation under a compressive force into a block of a desired form.
Fused quartz glasses are obtained primarily in a form of block or rod in a variety of methods known in the prior art. For example, U.S. Pat. No. 2,852,891 discloses a method in which finely divided granules of quartz are melted in an electric furnace and drawn out of the opening in the bottom of the furnace. Further, a method is disclosed in which finely divided particles of quartz are blown with a jet of high temperature flame at a rotating mandrel so that the molten quartz particles deposit successively to form a grown rod (see U.S. Pat. No. 3,128,166 and Japanese Patent Publication No. 46-42111).
Apart from the above mentioned processes applicable to the preparation of fused quartz glass starting with natural quartz, so-called synthetic quartz glass is manufactured by the process, for example, disclosed in U.S. Pat. No. 2,272,432 according to which a volatile halogenated compound of silicon such as silicon tetrachloride is subjected to vapor-phase hydrolysis in an oxyhydrogen flame and the fine particles of silica formed by the hydrolysis are blown at a rotating target of quartz glass heated at a temperature higher than the vitrification temperature by the heat of combustion to be deposited thereon forming a block on the target.
The above described method of vapor-phase hydrolysis or vapor-phase decomposition is of course applicable to other kinds of volatile silicon compounds in place of the silicon tetrachloride such as silane SiH.sub.4 (see Japanese Patent Kokai No. 50-21993) and volatile alkoxysilanes represented by the general formula R.sub.n Si(OR').sub.4-n, in which R and R' are each a hydrogen atom or a monovalent hydrocarbon group (see U.S. Pat. No. 2,852,891).
The above described methods of vapor-phase decomposition in a high temperature flame are advantageous when the fused quartz glass formed thereby is desired to be uniformly doped with a controlled amount of a dopant such as germanium, aluminum, iron, boron, phosphorus, zinc, tin and the like with an object to modify the refractive index or other properties of the fused quartz glass. That is, the vapor of a volatile compound of such a dopant element is admixed with the vapor of the silicon compound in a controlled proportion either in the flame or before entering the flame to be decomposed together so that the uniformity of the distribution of the dopant element is ensured throughout the glass block although it is optional that the above mentioned volatile compound of the dopant element is replaced with an oxide of the element prepared in advance.
Generally, the blocks of the fused quartz glass obtained in either of the above described methods are in the form of a rod having a circular cross section and the diameter of the rod is also limited according to the principle of the method. Therefore, it is sometimes desired that the quartz glass block to be subjected to a further fabrication by cutting or the like mechanical working means should have a larger diameter than the blocks as prepared by the above described methods or the block has a different cross sectional form than circular such as square, rectangular, elliptic and the like in order to minimize the loss of the material in the mechanical working. For example, a rod with a square cross section may be prepared by cutting and grinding of a circular rod but the loss of the material by the mechanical working is 36.3% by a geometrical calculation assuming that the length of the diagonal of the square is equal to the diameter of the circular rod. The loss of the material in the mechanical working is even larger when a rod of more complicated or irregular cross section is to be fabricated from a circular rod of the fused quartz glass.
Moreover, blocks of fused quartz glass as prepared by the above described methods cannot be free from striae, i.e. striated structure caused by the growth of the block, and local inclusion of foreign materials as well as strains caused by the inhomogeneity in the distribution of the foreign materials. These striae and the inhomogeneity in the composition of the glass are hardly removed by the annealing conventionally undertaken for removing the thermal stress.
Unless the above problems are satisfactorily solved, application of fused quartz glass is limited, especially, as a part of optical instruments such as lenses, prisms and the like.